An audio FM transmitter for wirelessly transmitting an audio signal (e.g., an audio signal of an audible frequency between 10 Hz and 20 kHz) in an ultra high frequency band such as the UHF band (between 300 MHz and 3 GHz) should generate a high-frequency reference signal. A known circuit for generating such a high-frequency signal is a frequency multiplier circuit. The frequency multiplier circuit can generate a high-frequency signal by extracting a high-frequency component from a low-frequency oscillation signal generated by a crystal resonator. However, in order to generate a desired high-frequency signal, the frequency multiplier circuit needs to be multistaged, which disadvantageously increases the size and cost of the circuit, so it is difficult to apply the multistaging of the frequency multiplier circuit to the audio FM transmitter that is highly required to achieve compact size and low cost.
In order to solve such a problem, a phase locked loop (PLL) circuit is widely used. For example, as shown in FIG. 3, a PLL circuit 100 includes a crystal resonator 101, a crystal oscillator 102, a reference frequency divider 103, a phase comparator 104, a programmable frequency divider 105, a loop filter 106 and a voltage-controlled oscillator (VCO) 107. In the PLL circuit 100, the crystal oscillator 102 outputs a reference signal by the oscillation of the crystal resonator 101, and the reference frequency divider 103 frequency-divides the reference signal. The phase comparator 104 outputs a phase difference signal depending on the phase difference between the output from the reference frequency divider 103 and the output from the programmable frequency divider 105, and the loop filter 106 integrates and converts the phase difference signal into a DC voltage signal. The voltage-controlled oscillator 107 performs oscillation using the DC voltage signal as a control signal to vary the oscillation frequency. The programmable frequency divider 105 frequency-divides the oscillation output branched from the output of the voltage-controlled oscillator 107 at a frequency division ratio set therein and outputs the division result to the phase comparator 104 (e.g., see FIG. 1 in Patent Document 1).
Conventionally, when the above-described PLL circuit 100 is applied to an audio FM transmitter, an audio signal is FM-modulated by superimposing a modulating signal in accordance with the audio signal onto the voltage-controlled oscillator 107, as shown in FIG. 3. In other words, the voltage-controlled oscillator 107 performs oscillation at a center frequency (carrier frequency) in accordance with the DC voltage signal from the loop filter 106 and FM-modulates the oscillation output with the modulating signal to transmit the FM-modulated signal from an antenna.